The description of horses comes in Ashwamedha Yagna, Dashrath in Ramayana, The Bhagavad Gita with Krishna riding a Rath and Kathoupanishad describing the meaning of a chariot.

In scientific terms horses represents our senses which needs to be controlled.

Most Upanishad and Vedic seers, talk of the horse and not any other animal as the symbol of sacrifice. Horses are known for their speed, dynamism, faithfulness and devotion. For realization, speed, dynamism, faithfulness and devotedness all are necessary. But horses are also known for their chanchalta or the quality to get out of control if not tightened. (bidakna)

One needs to sacrifice the internal uncontrolled horses and keep the internal horses under control so that we can acquire the divine qualities of horses.

The Brhadaranyaka Upanishad starts with the description of the sacrificial horse used in the ashwamedha sacrifice. It means that no spiritual Yagna is complete unless we learn to control our senses.

Kathoupanishad (1.3.4-7) Talks about the importance of controlling the mind and the senses. It describes the body as the chariot; the Self is its master; intellect is the charioteer, the mind as the reins, the senses as the horses and the sense-objects as the paths.

Number of horses also have different depictions: five in kathoupanishad (five senses motor or sensory); ten in Ramayana (Dashrath, who has a control, over his ten senses), seven in Lord Sun (Sapta-vaahanah — “One Who has the vehicle of seven horses.” Lord Sun, representing control over the mind all seven days of the week)

The imagery associated with the Bhagavad Gita is that of a chariot with four or five white horses. Arjuna is inside the chariot and the chariot is being driven by Lord Krishna. Here the human body represents the chariot, Arjun the individual soul and Krishna the Spirit or the Supreme Soul. That chariot has three wheels (Satwa, Rajas, and Tamas); has three kinds of motion (upwards or downwards or transversely, implying superior, inferior, and intermediate birth as brought about by acts); four horses apart from senses also represent the time, predestiny, will of the deities, and one’s own will. It has three naves (white, black, and mixed, implying good acts, evil acts and acts that are of a mixed character).

Vidur also talked about the same in vidur niti. He said to Dhritarashtra that O king, man’s body is just like a Ratha (chariot), intelligence (Buddhi) like a charioteer (Saarathi) and senses are its horses. One, who controls all these three, travels happily in this world, just like the charioteer who has controlled the horses of the chariot)